This invention relates generally to an electronic tone generating system, and more particularly to such a system for effectively reproducing a plurality of selected bell and chime tones of improved tonal quality.
The quality of musical tones produced by an instrument is generally determined by three basic properties of sound: pitch, tone color, and dynamics. Pitch, the "highness" or "lowness" of sound, depends on the speed or rate of the vibrations. The smaller the vibrating body, the faster the vibrations and the higher the sound (e.g., a piccolo encloses a smaller tube of vibrating air than does a trombone). As is well known, if one blows across the top of a bottle as it is filled up with water, the sound becomes higher as the vibrating column of air above the water becomes shorter.
The phenomenon of octaves has to do with the remarkable fact that strings and other sound-producing bodies tend to vibrate not only along their full length but also simultaneously in halves, quarters, and so forth. Acoustical physicists call these fractional vibrations "partials," while musicians call them "overtones." The sound of the overtones is very, very much softer than that of the fundamental note. But when a second string, half the length of the first, vibrates it also reinforces an overtone of the first (full-length) string. The ear receives this as a kind of "duplication."
Tone color, that indescribable quality of sound, depends on the amount and proportion of the overtones. In a flute, for example, the air column happens to vibrate largely along its total length and not much in halves or quarters, whereas violin strings vibrate simultaneously in many subsegments. This is what seems to account for the "white" tone color of the flute and the "rich" tone color of the violin.
Dynamics or loudness depends on the amplitude of the vibration, that is, on how far or hard the string or air column vibrates. For example, in a guitar, loudness depends on how many sixteenths or thirty-seconds of an inch the string flares out when it is plucked. The harder it is plucked, the louder the sound, of course. Players of wind instruments control dynamics by the wind pressure that they produce by blowing.
Generating sound by electronic methods on the other hand requires, first, the development of an electrical wave, and second, a means by which its energy can be used to produce audible sensations. To generate sounds having specific pitches and tones, the electrical waves have to be modified accordingly. Basically, this is what an electronic organ does. A basic electrical wave, such as a sine wave or a more complex saw tooth or pulse wave, is typically fed to a wave shaping network designed to produce an electrical wave which, when amplified and applied to a speaker, produces a sound having a specific pitch and tone.
Previous attempts to generate a bell-tone of substantial tonal quality have not been successful primarily because of their inability to generate the proper frequencies, or partials, contained within the bell-tone, and also because of their inability to individually control the attack and decay of the amplitude levels of the various frequencies contained therein. A bell-tone is essentially a "complex tone", that is, a sound wave produced by the combination of simple sinusodial components of different frequencies. Faithful reproduction of a bell-tone requires that the amplitude, attack, and decay of each one of the frequencies or partials involved in the bell-tone are able to be dynamically and interdependently changed based on which partial or related frequency the particular frequency is in relation to the fundamental frequency of the bell-tone.
The fundamental tone or frequency is variously described as the normal pitch of a musical tone, or the lowest frequency component of a complex waveform. As noted previously, a complex tone is made up of many simple sinusodial physical components of different frequencies. Each partial is, in turn, a sound sensation component that is distinguishable as a simple tone, cannot be further analyzed by the ear, and contributes to the character of the complex sound. The frequency of a partial may be higher or lower than the fundamental frequency and may be an integral multiple or submultiple of the fundamental frequency, as contrasted with a "harmonic" which is an integral multiple of the fundamental frequency. Therefore, in order to accurately reproduce a bell-tone of such tonal quality that the average listener could not distinguish the electronic bell-tone from a "real" bell sound, one is required not only to generate many frequencies, each related to the fundamental frequency of the bell, but also is required to individually control each one of the related frequencies as to its amplitude, attack, and decay.
An early attempt at incorporating the effects of attack and decay in a digital musical instrument is disclosed by Whitefield in U.S. Pat. No. 4,119,006. By appropriately scaling the digitally synthesized waveform information at the leading and trailing portions of the waveform envelope, Whitefield produces two attack and decay periods with only one of each resulting in the normal audible effect. One problem with such a system, as it pertains to the generation of bell-tones, however, is that it lacks the capacity for producing the characteristic "strike" of a bell since it indeed requires a predetermined length of time before the tone produced reaches its fullest intensity after the key has been depressed. In contradistinction, a real bell-tone has virtually no "attack" since it is dependent for its full intensity upon the percussive strike of its clapper.
Control of the "decay," or the length of time it takes for a tone to fade away after the playing key is released, has also been attempted in prior art devices with varying success. For example, Deutsch in U.S. Pat. No. 4,387,622 discloses a musical tone generator with independent time varying harmonics. A plurality of data words corresponding to the amplitudes of a corresponding number of evenly spaced points defining the waveform of one cycle of a musical signal are transferred sequentially from a note register to a digital-to-analog converter in repetitive cycles at a rate proportional to the pitch of the tone being generated. Thereafter, Deutsch discloses apparatus for approximating prespecified harmonic-time curves by piece wise segments of exponential functions. It is apparent, however, that such independent time varying harmonics are incapable of producing the required interdependency of a "real" bell-tone.
Electrical synthesis of a mechanical bell is also disclosed in U.S. Pat. No. 4,401,975--Ferguson. Circuit means are provided for synthesizing the sounds of a mechanical bell by combining the three most significant frequencies of the bell to be synthesized and modulating them with a decaying exponential control signal which is derived from a clock signal having a pulse repetition rate equal to the stroke repetition rate of the bell being synthesized. In a similar manner, Ferguson discloses in U.S. Pat. No. 4,437,088 an electronic circuit for simulating the sound of a percussive bell struck at a predetermined repetition rate. Both Ferguson patents, however, are directed to the type of bell that is ordinarily used as a household door bell. Accordingly, such devices are not suitable for the duplication of tonal characteristic of bells such as cast bells and chimes.
The advent of microprocessors has also enabled electronic devices to more faithfully reproduce musical instruments. For example, Budelman discloses in U.S. Pat. No. 4,409,877 a microprocessor-controlled electronic tone generating system for reproducing organ tones having improved harmonic content which includes a first group of tone generators having output frequencies defining a first musical scale, and second group of tone generators having output frequencies defining a second musical scale offset with respect to the first. The first group of tone generators is responsive to a keyboard operation (e.g., the actuation of a particular key) for generating the fundamental of the desired musical note as well as a first set of harmonic output frequencies. Likewise, the second group of tone generators is responsive to the same keyboard operation for reproducing a second set of harmonic output frequencies substituting for selected harmonic frequencies of the first set which fall outside predetermined error limits. In such a manner, the device simulates pipe organ sounds with thirty-two harmonics and two scales, the second scale reproducing "truer" 7th, 11th, 13th, 14th, 21st, 22nd, 25th, 26th, 28th and 31st harmonics. While such a device is capable of reproducing pipe organ voices with considerable accuracy, without inordinately increasing the number of tone generators in the system, it is nevertheless silent as to its applicability in the faithful reproduction of a variety of bell-tones. Furthermore, the mere reduction of the number of tone generators used to reproduce pipe organ voices, and correction for errors caused thereby, does not suggest the interdependent control of amplitude, attack, and decay of component partials in a complex tone such as bell-tone.